1. Field of the Invention
The present invention relates to a zoom lens capable of selecting and setting a focal length as desired within a predetermined range of the focal length, which is also capable of achieving a wide angle of view at the wide-angle end and a high changing magnification rate in addition to a small size and a high performance; it specifically relates to a zoom lens suitable for a camera using an electronic photographing means such as a digital camera and a video camera, which is also applicable to a film-based camera using a silver halide film, and a camera and a personal digital assistant using the above zoom lens.
2. Related Art
The traditional camera using a long established silver halide film, that is, the film-based camera has been replaced by the so-called digital camera or electronic camera, which photographs an object by a solid imaging device such as a CCD (charge coupled device), acquires image data of the object as a still image or an animated image, and records the data in a digital format such as a nonvolatile semiconductor memory represented by a flash memory. This type of camera has been wide spread as a single camera, as well as having been mounted on mobile phones and other personal digital assistants; it has also been applied to a new and novel use to the film-based camera.
The market for such a digital camera has been huge, and the users' demands for the digital camera range over many divergences. Among them, a higher image quality and a smaller size are always demanded by the users, and they form a great weight of the users' demands. Thus, both a higher performance and a smaller size are demanded also to the zoom lens used as a photographing lens.
Here, in view of seeking a smaller size, it is necessary to shorten a whole lens length of the zoom lens, that is, a distance between a lens surface on the most object side and an image surface. In view of seeking a higher performance, it is necessary that the zoom lens possesses a resolving power corresponding to an imaging device of at least about 8-10 million pixels, over the whole zooming range.
Besides, many users desire a wider angle of view of the photographing lens; and the half angle of view at the short focal end of the zoom lens, namely, the half angle of view at the wide-angle end thereof is desired to be 38 degrees or more. Among specialist photographers, namely, professional photographers, or lovers of photographs being called high amateurs, who have highly professional knowledge and techniques being comparable to those of the professionals, not a few desire a wider half angle of view of 42 degrees or more. The 38 degrees and 42 degrees in the half angle of view correspond to 28 mm and 24 mm in the focal length, respectively, when they are converted into the focal length in the 35 mm film-based camera using the 35 mm (the so-called Leica) silver halide film.
There are quite a few types conceivable as the zoom lens for the digital camera. The following zoom lens can be quoted as a type suitable for a smaller side. The zoom lens includes a first lens group having a negative refracting power, a second lens group having a positive refracting power, a third lens group having a positive refracting power, which are disposed in order from the object side, and an aperture stop that moves in one united body with the second lens group on the object side of the second lens group, wherein the second lens group moves monotonously from the image side toward the object side along with changing magnification from the wide-angle end toward the telephoto end, and the first lens group moves so as to correct the fluctuations of an image surface position along with the changing magnification.
Among such zoom lenses, there is a well-known configuration having united surfaces at two places of the second lens group, in order to perform a satisfactory correction of axial chromatic aberrations and power chromatic aberrations, or to control a deterioration of image-forming performances due to a decentering among the lenses.
For example, the zoom lens that includes two pairs of cemented lenses in the second lens group is disclosed in JP2001-281545A, JP2003-107348A, JP2003-241091A, and JP2006-113554A. And, for example, the zoom lens that includes three cemented lenses in the second lens group is disclosed in JP2004-102211A, JP2004-325975A, JP2005-24804A, JP2005-37576A, and JP2006-39523A.
JP2001-281545A discloses, in the embodiment 1 through the embodiment 8 thereof, that a satisfactory correction of axial chromatic aberrations and power chromatic aberrations can be achieved by using two pairs of cemented lenses to the second lens group. However, the half angle of view is smaller than 34 degrees, which cannot be said sufficient in view of seeking a wider angle of view.
In the same manner, JP2003-241091A discloses, in the embodiments 11 and 12 thereof, that a satisfactory correction of axial chromatic aberrations and power chromatic aberrations can be achieved by using two pairs of cemented lenses to the second lens group. Also in this case, the half angle of view is lower than 34 degrees, which is insufficient in view of seeking a wider angle of view.
JP2004-102211A discloses, in the embodiments 11, 15, and 17 thereof, a configuration that applies three cemented lenses to the second lens group in consideration for a deterioration of image-forming performances due to a decentering among the lenses. Also in this configuration, the half angle of view is lower than 33 degrees, which is insufficient in view of seeking a wider angle of view.
In the same manner, JP2004-325975A, JP2005-37576A, and JP2006-39523A disclose a configuration that applies three cemented lenses to the second lens group in consideration for a deterioration of image-forming performances due to a decentering among the lenses, whereby the whole size of the zoom lens can be made up relatively small. However, the half angle of view is about 30 to 33 degrees, which is also insufficient in view of seeking a wider angle of view.
JP2003-107348A discloses a zoom lens that applies two pairs of cemented lenses to the second lens group to thereby achieve a satisfactory correction of axial chromatic aberrations and power chromatic aberrations, and attain a relatively wide half angle of view of about 39 degrees, which, however, cannot be said sufficient to the demand for the half angle of view of 42 degrees or more.
JP2005-24804A discloses a zoom lens that applies three cemented lenses to the second lens group to thereby achieve a satisfactory correction of axial chromatic aberrations and power chromatic aberrations, and attain a relatively wide half angle of view of about 39 degrees, which, however, cannot be said sufficient to the demand for the half angle of view of 42 degrees or more, in the same manner as the case with the JP2003-107348A.
JP2006-113554A discloses a zoom lens, in a part of the embodiments thereof, that uses two pairs of cemented lenses to the second lens group to attain a wide half angle of view of 43 degrees or more, which is a sufficiently wide angle of view.
However, in order to achieve a satisfactory correction of power chromatic aberrations, a low dispersive glass of the Abbe number of 80 or more is used to the negative lens of the first lens group. The low dispersive glass of the Abbe number of 80 or more is the so-called special low dispersive glass, the material cost thereof is high, the degree of processing difficulty is high, and the yield becomes decreased as the lens size becomes increased. In the usual processing of lenses, a process is included which cleans individual lenses by an ultrasonic cleaning, for example. However, the lens of a special low dispersive glass cannot be cleaned by the ultrasonic cleaning, which is simple and widely used, and the hand work called hand cleaning with a cloth becomes inevitably necessary. Accordingly, as the lens size becomes larger, the processing of the lens becomes more difficult, and the yield becomes lowered. Therefore, the use of it to such a place as the first lens group, where the diameter of lens is apt to increase, sharply raises the cost of the lens system, which is not preferred.
Accordingly, there is a need for a zoom lens that various aberrations are effectively controlled without a remarkable increase of the manufacturing cost, a sufficient wide angle of view is achieved at the wide-angle end, and a smaller size as well as a higher resolving power is realized. There is also a need for a camera and a personal digital assistant using such a zoom lens.